1. Field of the Invention
The invention relates to the fabrication of circuit boards having circuit patterns formed in plural planes. Specifically, the invention relates to selection of substrate materials for accommodating said fabrication.
2. The Prior Art
A number of processes are available from the prior art for the fabrication of circuit boards of the type comprising a dielectric substrate having a pattern of electrical path conductors thereon. For example, in one process, a conductive laminate is provided over the dielectric and coated with an etchant resist material which decomposes when exposed to light of particular wavelength. Upon shielding the resist by a pattern of opaque circuit paths, the decomposed resist material is removed by an etchant except for that which is covered by the pattern of resist material.
The application of metal by electroplating to a sensitized substrate is another technique known by those skilled in the art. However, etching or electroplating processes are costly, step intensive, and limited as to the circuitry density achievable thereby.
A further approach consists of laminating a dielectric substrate with a layer of metal, and then striking the metal with a stamping die. The face of the die is impressed with a pattern of circuitry which shears the metal laminate into a pattern of circuit conductors, separated from one another along the lines of shear. The die also indents the substrate forming permanent indentations into which separated portions of the metal laminate are recessed.
Such a technique is taught by U.S. Pat. No. 3,628,243. The primary shortcoming of the above approach rests as a consequence of the stamping operation. The die profile, in shearing and recessing the separated metal portions into the substrate, causes complementary protrusions to emerge from the underside of the substrate; the protrusions being comprised of substrate material which is displaced by the penetration of the die. As a result, circuitry on such boards must be restricted to one side of the substrate; the opposite side of the substrate being rendered useless by the protrusions made to emerge therefrom. While one sided circuitry is functional, it would obviously be economically preferable to use both substrate sides for circuitry. Even for those applications where only one sided circuitry is required, it would be desirable to preserve geometrical uniformity on the underside of the substrate to readily facilitate packaging.